1. Field of the Invention
The present invention relates to a MOS type integrated circuit, in particular relates to a basic oscillator when carrying out analogue signal processing.
2. Description of the Related Art
In recent years, due to the increase of digital devices and advancement in digital signal processing technology, CMOS integrated circuits suitable for digital signal processing have gained a large share of the semiconductor market. However, since video and audio signals are analogue signals, analogue signal processing can be more readily carried out, or, even when being digitally processed, analogue circuits are necessary for A/D converter, D/A converter, filtering processing carried out before and after that, oscillators for clock generation and the like. It has been said that, for analogue circuits, bipolar technology is suitable, whereas CMOS technology is unsuitable except for some circuits such as analogue switches and sample-hold circuits. However, in addition to that bipolar and BiCMOS processing are rather expensive, for this reason, it is strongly desired that CMOS integrated circuits are equipped with analogue circuits and digital circuits on one chip, and there has been an increase in the development of circuits for processing analogue signal with CMOS circuit.
The "oscillator" with the CMOS has been used much as clock generators or element circuits of PLL in digital signal processing. As representative oscillators as such, there are "ring oscillators" in which CMOS inverters are arranged in multi-stages to form a ring. Among them, one example of oscillators capable of controlling oscillation frequency is disclosed in Japanese Patent Laid-open Application No. Hei-4-188910, main portion of the oscillator thereof is constituted as such shown in FIGS. 14A and 14B.
In this ring oscillator, as shown in FIG. 14A, inverter circuits of I1, I2 . . . In are connected in turn in multi-stages, and output of the final stage of the inverter circuit In is returned to the input of the first inverter circuit I1, thereby a loop of ring is formed.
Each of the inverter circuits I1, I2 . . . In, as shown in FIG. 14B, consists of 4 field effect transistors M41 to M44. Transistors M41 and M42 are formed of N-channel MOS transistors, while transistors M43 and M44 are formed of P-channel MOS transistors. Gates of transistors M42 and M43 are connected to be used as an input terminal, while drains of transistors M42 and M43 are connected to be used as an output terminal. Further, drain of transistor M44 and source of transistor M43, source of transistor M42 and drain of transistor M41 are connected respectively, at the same time, source of transistor M44 is connected to a terminal of supply voltage, and source of transistor M41 is connected to earth point.
Each gate of transistors M41 and M44 of each of the inverter circuits 11, I2 . . . In is connected to terminals T1 and T2 for controlling frequencies, respectively. Output signals can be extracted from any one output of each inverter circuit of I1,I2 . . . In, and are outputted through an another inverter which is connected to the loop circuit.
In this ring oscillator, a pair of transistors M41, M42 and a pair of transistors M43, M44 of each inverter circuit are made on/off in a complementary manner depending on signal level supplied to the input terminal IN. Therefore, signal levels at the input terminal In and the output terminal OUT in each inverter circuit are to be inverted. Since inverter circuits are connected in series of multiple stages, such inversions are propagated successively. Due to its ring structure, by returning to its starting point and further enhancing inversion, finally the inversion motion results in oscillation of a whole loop.
Such an oscillation frequency is determined by the delay time of inversion signals between input and output of each inverter circuit. When a delay time for an inverter is td, an oscillation frequency fosc is EQU fosc=1/(Ntd) (1)
Respective gate voltages of transistors M41 and M44 restrict current supplied to these field effect transistors. Thereby, current supplied from transistor M44 to M41 during inversion of inverter circuit is also restricted, thus delay time td also varies. Therefore, if voltages supplied to control terminals T1, T2 are varied, delay times of inversion signals in respective inverter circuits will change all at once.
Thus, when the delay time of inversion signals varies, since the time at which propagation of inversion returns after one round of the loop also varies, the oscillation frequency also varies according to equation (1). That is, when the delay time td per a stage is made fast, the frequency becomes high, and when the delay time td is made slow, the oscillation frequency becomes low, thus the oscillation frequency can be easily controlled by varying the delay time of inversion signals.
A ring oscillator comprising of such an inverter circuit, when employed in a CMOS LSI which is equipped with analogue circuits and digital circuits on one chip, has the following disadvantages such as
1. The ring oscillator itself generates a noise like pulse to be liable to adversely affect on the other analogue circuits.
2. Since it is likely to be influenced by the noise of the power source and the like, there is much jitter (phase noise). In the inverter circuit, only at the instance of inversion, a relatively large current flows between the power source and ground.
Therefore, each time when the inverter reverses, according to resistances of power source line and ground line, pulse voltages are generated at power source and ground. When LSI contains analogue circuits, even if a measure is taken such as separation of a power source line and the like, this pulse voltage goes around to analogue circuit side due to common impedance of the power source line or capacitive coupling according to substrate and the like. This gives some influence to analogue circuits, thereby pulse noise is added to analogue signal to inevitably deteriorate its quality to some degree.
In particular, when oscillator output is employed as a certain reference signal in analogue signal processing, since ratio of frequency of this reference signal and that of the aforementioned pulse voltage has integer ratio, beat component is added on the signal and may be noise incapable of being separated by filter and the like. Further, wave form of inverter circuit constituting a ring oscillator becomes rectangular wave which swings fully between the power source and ground. Since this has spurious components (harmonic component) with high energy, if there is a part of high impedance in an analogue circuit, by plunging in as radiation, the signal quality tends to deteriorate.
On the contrary, even in digital circuits, at the instance when state transits, relatively large penetrating current flows between the power source and ground. Therefore, when an LSI contains digital circuits, for whole digital circuits, various inversions occur with timings of the edge of clock rise or fall signals, and a noise like pulse due to this is added on power source line or ground line. As described above, even if a measure such as a separation of power source line and the like is taken, this pulse voltage, due to common impedance of the power source line and due to capacitive coupling according to substrate, can not be avoided to go around to the power source/ground line of oscillator.
In the ring oscillator, because of oscillation with amplitude of voltage between a power source and ground, the noise added on the source voltage induces temporary variation of amplitude, and causes fluctuation to the delay times of the inverter which determines frequency. This finally becomes the jitter (phase noise) of oscillation frequency and deteriorates spectral purity of oscillation. Further, penetrating current like the pulse which is generated by the inverter circuit itself constituting the ring oscillator during inversion causes to generate pulse voltage on the power source/ground line which the inverter circuit itself is using, thereby that results in enhancement of the jitter (phase noise) of oscillation frequency.